Wireless communication systems are widely known in which base stations (BSs) form “cells” and communicate with user equipments (UEs) (also called subscriber or mobile stations) within range of the BSs.
In such a system, each BS divides its available bandwidth, i.e. frequency and time resources in a given cell, into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one “serving” or “primary” cell.
Modern wireless communication systems such as LTE and LTE-A are hugely complex and a full description of their operation is beyond the scope of this specification. However, for assisting understanding of the inventive concepts to be described later, some outline will be given of some of the features of LTE which are of particular relevance in the present invention.
Basic LTE Network Topology
The network topology in LTE is illustrated in FIG. 1. As can be seen, each UE 12 connects over a wireless link via a Uu interface to an eNB 11, and the network of eNBs is referred to as the eUTRAN 10.
Each eNB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities, including a Serving Gateway (S-GW 22), and a Mobility Management Entity (MME 21) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition, a PDN or Packet Data Network Gateway (P-GW) is present, separately or combined with the S-GW 22, to exchange data packets with any packet data network including the Internet. The core network 20 is called the EPC or Evolved Packet Core.
Machine Type Communication (MTC) and Machine-to-Machine (M2M) Communication
Machine-to-Machine (M2M) communication, usually referred to in the context of LTE as Machine Type Communication (MTC), is a form of data communication which involves one or more entities that do not necessarily need human interaction; in other words the ‘users’ may be machines.
MTC is different from current communication models as it potentially involves very large number of communicating entities (MTC devices) with little traffic per device. Examples of such applications include: fleet management, smart metering, product tracking, home automation, e-health, etc.
MTC has great potential for being carried on wireless communication systems (also referred to here as mobile networks), owing to their ubiquitous coverage. However, for mobile networks to be competitive for mass machine-type applications, it is important to optimise their support for MTC. Current mobile networks are optimally designed for Human-to-Human communications, but are less optimal for machine-to-machine, machine-to-human, or human-to-machine applications. It is also important to enable network operators to offer MTC services at a low cost level, to match the expectations of mass-market machine-type services and applications.
To fully support these service requirements, it is necessary to improve the ability of mobile networks to handle machine-type communications.
In the LTE network illustrated in FIG. 2, a group of MTC devices 200 is served by an eNB 11 which also maintains connections with normal UEs 12. The eNB receives signalling from the MME 21 and data (for example, a request for a status report from a supervisor of the MTC devices) via the S-GW 22.
Thus, there is a MTCu interface analogous to the Uu interface, and the MTC devices will be served in a similar way to normal user equipments by the mobile networks. When a large number of MTC devices connect to the same cell of a UMTS RNS or an LTE eNB, each of the devices will need resources to be allocated to support the individual devices' applications even though each MTC device may have little data.
In the remainder of this specification, the term “UE” includes “MTC device” unless otherwise demanded by the context.
For assisting understanding of the inventive concepts to be described later, some outline will be given of some specific aspects or features of LTE which are of particular relevance in the present invention.
OFDMA and SC-FDMA
In the downlink of an LTE system, in other words the direction of transmission from the base station (eNB) towards the user equipments (UEs), individual OFDM subcarriers or sets of subcarriers are assigned to different user equipments. The result is a multi-access system referred to as OFDMA (Orthogonal Frequency Division Multiple Access). By assigning distinct frequency/time resources to each user equipment in a cell, OFDMA can substantially avoid interference among the users served within a given cell.
The UEs are allocated a specific number of subcarriers for a predetermined amount of time. An amount of resource consisting of a set number of subcarriers and OFDM symbols is referred to as a resource block (RB) in LTE. RBs thus have both a time and frequency dimension. Allocation of RBs is handled by a scheduling function at the eNB.
The uplink in an LTE wireless communication system employs a variant of OFDMA called Single-Carrier FDMA (SC-FDMA). Essentially, SC-FDMA is a linearly precoded OFDMA scheme, involving an additional DFT step before the conventional OFDMA processing. Access to the uplink by multiple UEs is enabled by assigning to each UE a distinct set of non-overlapping sub-carriers. This allows a single-carrier transmit signal, reducing the peak-to-average power ratio (PAPR) in comparison with OFDMA.
Frame Structure and Resource Blocks
In a wireless communication system such as LTE, data for transmission on the downlink is organised in OFDMA frames each divided into a number of sub-frames. Various frame types are possible and differ between frequency division duplex (FDD) and time division duplex (TDD) for example.
FIG. 3 shows a generic frame structure for LTE, applicable to the downlink, in which the 10 ms frame is divided into 20 equally sized slots of 0.5 ms. A sub-frame SF consists of two consecutive slots, so one radio frame contains 10 sub-frames.
The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM symbols, as shown in FIG. 4. Each element in the resource grid is called a resource element (RE) and each resource element corresponds to one symbol.
For each transmission time interval of 1 ms, a new scheduling decision is taken regarding which UEs are assigned to which time/frequency resources during this transmission time interval, the scheduling being made in units of resource blocks (RB). As shown in FIG. 4, one resource block is usually defined as 7 consecutive OFDM symbols in the time domain and 12 consecutive sub-carriers in the frequency domain. Several resource blocks may be allocated to the same UE, and these resource blocks do not have to be contiguous with each other. Scheduling decisions are taken at the eNB, using a scheduling algorithm which takes into account the radio link quality situation of different UEs, the overall interference situation, Quality of Service requirements, service priorities, etc.
Channels
In LTE, several channels for data and control signalling are defined at various levels of abstraction within the system. FIG. 5 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them. For present purposes, the channels at the physical layer level are of most interest.
On the downlink, user data is carried on the Physical Downlink Shared Channel (PDSCH). There are various control channels on the downlink, which carry signalling for various purposes including so-called Radio Resource Control (RRC), a protocol used as part of radio resource management, RRM. In particular this signalling comprises the Physical Downlink Control Channel, PDCCH (see below).
Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH). By means of frequency hopping on PUSCH, frequency diversity effects can be exploited and interference averaged out. The control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel state information (CSI), as represented for example by channel quality indication (CQI) reports, and scheduling requests.
PDCCH and DCI
In LTE, both the DL and UL are fully scheduled since the DL and UL traffic channels are dynamically shared channels. This means that PDCCH must provide scheduling information to indicate which users should decode the physical DL shared channel (PDSCH) in each sub-frame and which users are allowed to transmit on the physical UL shared channel (PUSCH) in each sub-frame. PDCCH is used to carry scheduling information—called downlink control information, DCI—from base stations (called eNBs in LTE) to individual UEs. Conventionally, one PDCCH message contains one DCI format. This is often intended for one individual UE, but some messages are also broadcast (e.g. intended for multiple UEs within a cell). Thus PDCCH can also contain information intended for a group of UEs, such as Transmit Power Control (TPC) commands. In addition the PDCCH can be used to configure a semi-persistent schedule (SPS), where the same resources are available on a periodic basis. The motivation for SPS is to support applications.
PDCCH is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where a control channel element corresponds to 9 resource element groups (REG). Each REG in turn occupies four of the Resource Elements (REs) shown in FIG. 4.
More particularly PDCCH contains:                the resource allocations for the downlink transport channel DL-SCH        Transmit Power Control (TPC) commands for PUCCH and the uplink transport channel UL-SCH; these commands enable the UE to adjust its transmit power to save battery usage        Hybrid-Automatic Repeat Request (HARQ) setup information        MIMO (see below) precoding information.        
A cyclic redundancy check (CRC) is used for error detection of the DCI. The entire PDCCH payload is used to calculate a set of CRC parity bits, which are then appended to the end of the PDCCH payload.
As multiple PDCCHs relevant to different UEs can be present in one sub-frame, the CRC is also used to specify which UE a PDCCH is relevant to. This is done by scrambling the CRC parity bits with a Radio Network Temporary Identifier (RNTI) of the UE. Various kinds or RNTI are defined, as explained in more detail below.
The size of the DCI depends on a number of factors, and thus it is necessary that the UE is aware of the size of the DCI, either by RRC configuration or by another means to signal the number of symbols occupied by PDCCH.
Depending on the purpose of the DCI message, different DCI formats are defined. The DCI formats include:                Format 0 for transmission of uplink shared channel (UL-SCH) allocation        Format 1 for transmission of DL-SCH allocation for Single Input Multiple Output (SIMO) operation        Format 1A for compact transmission of DL-SCH allocation for SIMO operation or allocating a dedicated preamble signature to a UE for random access        Format 3 and format 3A for transmission of TPC command for an uplink channel.        
DCI Formats 3 and 3A carry multiple power control bits representing multiple power control commands, each power control command being intended for a different UE. The main application of interest for Formats 3 and 3A is to support SPS in the uplink (since UE specific PDCCH DCI formats to carry power control commands are not then required).
Further details of the full set of DCI formats already defined in LTE can be found In the document 3GPP TS36.212 “Evolved Universal Terrestrial Radio Access (E-UTRA): Multiplexing and channel coding”, hereby incorporated by reference. As an example, format 0 is specified as follows:
DCI Formatfieldssizedescription‘Format0’DCI Format—‘Format0’: indicates the DCIformat to the UEAllocationType1-bitResource allocation header:type0, type1 (for uplinkfrequency hopping)AllocationvariableResource block assignment/allocation: indicates the startingRB as well as the number ofcontiguous RBs allocated to theUEHoppingFlag1-bitPUSCH hopping flag (for uplinkfrequency hopping)ModCoding5-bitsModulation, coding scheme andredundancy versionNewData1-bitNew data indicator (a newtransmission is to be sent)TPC2-bitsPUSCH TPC command foradapting the UE's transmitpowerCShiftDMRS3-bitsCyclic shift for an uplinkdemodulation reference signalDM RSCQIReq1-bitCQI request: requests UE tosend a channel quality indicationDAI2-bitsDownlink assignment index(TDD only)ULIndex2-bitsUL index (TDD only)
Since, as already mentioned, multiple UEs can be scheduled within the same sub-frame, conventionally therefore multiple DCI messages are sent using multiple PDCCHs.
The format to be used depends on the purpose of the control message. For example, DCI format 1 is used for the assignment of a downlink shared channel resource when no spatial multiplexing is used (i.e. the scheduling information is provided for one code word transmitted using one spatial layer only). The information provided enables the UE to identify the resources, where to receive the PDSCH in that sub-frame, and how to decode it. Besides the resource block assignment, this also includes information on the modulation and coding scheme and on the hybrid ARQ protocol used to manage retransmission of non-received data.
A UE needs to check all possible combinations of PDCCH locations, PDCCH formats, and DCI formats and act on those message with correct CRCs (taking into account that the CRC is scrambled with a RNTI). To reduce the required amount of ‘blind decoding’ of all the possible combinations, for each UE a limited set of CCE locations is defined where a PDCCH may be placed. The set of CCE locations in which the UE may find its PDCCH is called the “search space”. In LTE, separate UE-specific and common search spaces are defined, where a dedicated search space is configured for each UE individually, while all UEs are informed of the extent of the common search space.
RNTIs
RNTIs or Radio Network Temporary Identifiers, mentioned earlier, are used by the eNB to scramble the CRC applied to the PDCCH payload. Types of RNTI currently defined in LTE include the following.
P-RNTI (Paging RNTI):
To receive paging messages from E-UTRAN, UEs in an idle mode monitor the PDCCH channel for a P-RNTI value used to indicate paging. If the terminal detects a group identity used for paging (the P-RNTI) when it wakes up, it will process the corresponding downlink paging message transmitted on the PCH.
SI-RNTI (System Information RNTI):
The presence of system information on DL-SCH in a sub-frame is indicated by the transmission of a corresponding PDCCH marked with a special System Information RNTI (SI-RNTI). This PDCCH message indicates the transport format and physical resources (set of resource blocks) allocated for system-information transmission.
M-RNTI (MBMS RNTI):
This is used in Multimedia Broadcast Multicast Services (MBMS), a point-to-multipoint transmission scheme available in LTE.
RA-RNTI (Random Access RNTI):
The RA-RNTI is used on the PDCCH when Random Access Response (RAR) messages are transmitted, to identify which time-frequency resource was utilized by the UE to transmit a Random Access preamble. In the event of a collision when multiple UEs select the same signature in the same preamble time-frequency resource, they each receive the RAR message.
C-RNTI (Cell RNTI):
The C-RNTI is used by a given UE while it is in a particular cell, after it has successfully joined the network by performing a network entry process with the eNB of that cell. The C-RNTI is used for normal scheduling of downlink resources for the UE, also called dynamic scheduling as opposed to semi-persistent scheduling (see below).
TC-RNTI:
If a UE does not have an allocated C-RNTI, then a Temporary C-RNTI (TC-RNTI) is used for further communication between the terminal and the network. Once the UE has completed the network entry process, the TC-RNTI is changed to a C-RNTI.
SPS-C-RNTI (Semi-Persistent Scheduling C-RNTI):
This form of RNTI is used in SPS (see below). For the configuration or reconfiguration of a persistent schedule, RRC signalling indicates the resource allocation interval at which the radio resources are periodically assigned to a specific UE. Specific transmission resource allocations in the frequency domain, and transmission attributes such as the modulation and coding scheme, are signalled using the PDCCH. The actual transmission timing of the PDCCH messages is used as the reference timing to which the resource allocation interval applies. When the PDCCH is used to configure or reconfigure a persistent schedule, it is necessary to distinguish the scheduling messages which apply to a persistent schedule from those used for dynamic scheduling. For this purpose, a special identity is used, known as the Semi-Persistent Scheduling C-RNTI (SPS-C-RNTI), which for each UE is different from the C-RNTI used for dynamic scheduling messages.
TPC-PUCCH-RNTI (Transmit Power Control-Physical Uplink Control Channel-RNTI) and TPC-PUSCH-RNTI (Transmit Power Control-Physical Uplink Shared Channel-RNTI):
The power-control message is directed to a group of terminals using an RNTI specific for that group. Each terminal can be allocated two power-control RNTIs, one for PUCCH power control and the other for PUSCH power control. Although the power control RNTIs are common to a group of terminals, each terminal is informed through RRC signaling which bit(s) in the DCI message it should follow.
Further details of RNTIs available in LTE are given by the document 3GPP TS 36.213: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, hereby incorporated by reference.
SPS
Semi-Persistent Scheduling, SPS, schedules resources for UEs on an ongoing basis and thereby reduces control channel overhead for applications that require persistent radio resource allocations such as VoIP (Voice over Internet Protocol). In LTE, both the DL and UL are fully scheduled as already mentioned so that without SPS, every DL or UL physical resource block (PRB) allocation must be granted via a PDCCH message. Note that although retransmissions on PUSCH can be made autonomously without an explicit UL grant, the first transmission would require a grant. This works well with large packet sizes and only a few users to be scheduled each sub-frame. However, for applications that require persistent allocations of small packets, the control channel overhead due to scheduling information can be greatly reduced with SPS. In SPS, the eNB defines a persistent resource allocation that a user should expect on the DL or can transmit on the UL. This can also be highly beneficial for MTC for example, where the MTC devices may be expected to transmit a small amount of data at fixed intervals.
On the other hand, SPS as currently defined has various limitations as will be explained later.
R-PDCCH
FIGS. 1 and 2 show network topologies in which UEs and/or MTC devices communicate directly with a eNB. However, it is likely that practical LTE deployments will employ relay nodes (RNs) intermediate between UEs or MTC devices and the eNB providing the cell.
A new physical control channel, called the relay physical downlink control channel (R-PDCCH), may be used to dynamically or semi-persistently assign resources, within the semi-statically assigned sub-frames, for the relay physical downlink shared channel (R-PDSCH). The R-PDCCH is also used to dynamically or semi-persistently assign resources for the relay physical uplink shared channel (R-PUSCH).
R-PDCCH may be transmitted on a subset (including up to all of) of the OFDM symbols of the sub-frames assigned for the backhaul link (PDSCH). It is transmitted starting from an OFDM symbol within the sub-frame that is late enough so that the RN can receive it. R-PDCCH may be used to assign DL resources in the same sub-frame and/or in one or more later sub-frames; it may be also be used to assign UL resources in one or more later sub-frames.
Further details of R-PDCCH can be found in the LTE standards document 3GPP TS 36.216: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer for relay operation”, hereby incorporated by reference. In the remainder of this specification, references to PDCCH are to be understood as including R-PDCCH unless the context demands otherwise.
DRX and DTX
Discontinuous Reception (DRX) and Discontinuous Transmission (DTX) are techniques for saving power at the UE, and are also highly relevant to MTC. Both DRX and DTX involve reducing switching off the UE's transceiver periodically. Although the data throughput capacity is reduced in proportion to power saving, this is often not a problem for MTC devices having only a limited data capacity.
The eNB sets a cycle where the UE is operational for a certain period of time, during which all the scheduling and paging information is transmitted. Except in DRX mode, the UE's transceiver must be active to monitor PDCCH (to identify DL data).
Limitations of PDCCH and SPS
Currently in LTE, control channel messages (using PDCCH) may be transmitted to a UE from one or more serving cells. This control channel is typically used to indicate to the UE information about a downlink transmission that will occur on a downlink data channel (PDSCH) or to grant resources for transmission on an uplink data transmission (on PUSCH). In addition the PDCCH can be used to configure a semi-persistent schedule (SPS), where the same resources are available on a periodic basis. PDCCH can also contain information intended for a group of UEs. In particular Formats 3 and 3A carry multiple TPC bits, each intended for a particular UE. In general however, and particularly for scheduling other than SPS, a separate PDCCH is required for each UE.
A PDCCH transmission typically contains a payload of around 50 bits (including CRC), with additional channel coding to improve robustness to transmission errors. For some applications only small data packets are required, so the PDCCH payload may represent a significant overhead. This may be even more significant for some configurations of TDD, with a limited proportion of subframes allocated for DL transmission. In addition, there is a limit on the maximum number of PDCCH messages that can be transmitted at the same time (i.e. within the same subframe), which may be insufficient to support a large number of active UEs transmitting or receiving only small data packets.
One scenario where such control channel limitations may be significant is for Machine-To-Machine (M2M) Communication or Machine Type Communication (MTC). As a particular example, a sensor application may require small data packets (e.g. temperature readings) to be sent at short intervals from a large number of devices within one cell.
Meanwhile, semi-persistent scheduling (SPS) allows the resource allocation to be pre-configured. However, changing the resource allocation (including timing) of SPS for one UE requires a PDCCH message specifically for that UE.
In any case the current control channel arrangements intended for SPS suffer from a number of limitations, some of which are considered here:—                The availability of resources for SPS is limited to particular limited set of periodicities        The number of resource elements (REs) for SPS is fixed        The data rate (transport block size) for SPS is fixed        The modulation and coding scheme for SPS is fixed        
Scenarios where such control channel limitations may be significant, so that neither UE-specific PDCCH DCI formats or SPS are directly suitable, could include the following:—                Applications requiring regular transmission of small packets but with variable size (e.g. VoIP, where the packets may have one of a small set of sizes)        Applications requiring intermittent or irregular transmission of small packets of the same size (e.g. a sensor application sending a reading when the temperature changes)        Applications requiring regular transmission of small packets with the same size (e.g. VoIP) but where the variations in the radio channel mean that efficient channel adaptation requires variation of the transmission rate and/or location of the resource allocation in the frequency domain        Applications which could otherwise be supported by SPS but where the desired HARQ operating point leads to a high probability of retransmission, each retransmission requiring a PDCCH message.        
Therefore means for provision of efficient control channel functionality with low overhead for the above, for example by extending SPS functionality, is of significant interest.